1. Technical Field
The present invention relates to low pressure countergravity metal casting systems for countergravity casting molten metal articles under low pressure and more particularly to such systems having control systems for controlling the flow of the molten metal into the casting mold.
2. Description of Related Prior Art
Conventional low pressure casting systems typically comprise a casting mold supported above a furnace of molten metal and includes some means for forcing the metal from the furnace upwardly into a casting cavity within the mold. Some systems utilize pressurized gas to feed the molten metal while others employ an electromagnetic pump.
With either type of system it is important to precisely control the flow of the molten metal into the mold in order to achieve good castings. This is particularly true when casting thin-walled articles. Filling the mold cavity too fast tends to leave the thin sections of the cavity unfilled, whereas filling too slow allows for premature solidification producing porosity defects in the resultant cast article.
For those systems employing electromagnetic pump, the ideal manner in which the mold should be filled can be determined and expressed in terms of an ideal pressure of the pumped metal versus casting cycle time. Since the pump is electromagnetic, its pressure output is a function of the voltage applied to the pump. By simply controlling the voltage applied to the pump as a function of time, one known system attempts to conform the actual filling conditions with the desired ideal conditions. This time-dependent system, however, is deficient in that it fails to take into account the effects that varying metal temperature has on the pressure output of the pump as well as the changing relationship between the pressure output of the pump and the applied voltage as the pump wears and the varying relationship of the pressure output to the applied voltage as between different pumps. Furthermore, the accuracy of this type of control system is dependent upon the metal level starting out at the same level for each casting cycle. If the metal level starts out too high, then the thin sections of the mold cavity will likely not be filled and further the molten metal will likely penetrate the sand mold as well as produce unwanted flashes at the parting line of the mold.
Other control systems have been developed which address the deficiencies of the time-controlled system but which themselves suffer from various other deficiencies. For example, one control system is known in which induction sensors are positioned around the mold for detecting the actual level of the molten metal as it rises in the mold. The sensor then controls the output of the pump in order to conform the actual filling of the mold with an ideal metal level versus casting cycle time schedule. This system, however, can not be used when other metal objects are present in the mold cavity. Such is the case, for example, when casting cylinder blocks with cast-in-place metal cylinder liners. The liners interfere with the sensor's ability to detect and monitor the position of the metal in the mold.
Another known system monitors the temperature of the metal as it rises in the mold. The output of the pump is then controlled so that the actual metal temperature conforms with a predetermined ideal metal temperature versus casting time schedule. This control system, however, requires that each mold be fitted with numerous temperature sensors (i.e., thermal couples) which would be undesirable for production level casting and costly.
Accordingly, there is a need in the industry for a control system which can precisely control the output of an electromagnetic pump but yet is practical and suitable for production rate casting.